In high-tin lead-free solders, the solder solidifies with Sn "dendrites,"
much like the ice crystals that form on your car window in the winter.
Even eutectic Sn-Ag, Sn-Cu, and Sn-Ag-Cu alloys show dendrites, not the
fine, uniform eutectic structures that we're accustomed to for eutectic
Sn-Pb. (Take a look at the Sn Ag eutectic microstructure that I've attached
to the email for a look at what's typical.) So the graininess observed on
the surface of the joint is due to dendrite formation and is a fact of life
for these lead-free solders. It also means that the visual inspection
criteria are going to have to be different for lead-free solders. The size
scale of the graininess will depend to some extent on the cooling rate,
but, as far as I know, it not clear at this point what the relationship is
between the size scale of the dendrites and the reliability.
At 05:53 PM 08/01/2000 +0300, you wrote:
>Dave
>
>Compare your results with the excellent cover photo of Soldering & Surface
>Mount
>Technology Volume 12 Number 2 (2000) which has the caption SnAgCu soldered
SOIC
>joint showing typical surface roughness. I think you may find that high Sn
>alloys are just simply more light-diffractive than near-eutectic SnPb ones.
>Notwithstanding, you seem to be changing two parameters (alloy and flux
>type) at
>the same time. It may be better to change just one, so that you can isolate
>which is causing you grief. Also, may I suggest a metallographic section of
>such
>a 'faulty' joint to see whether it has bonded well?
>
>Brian
>
>Dave Crabbe wrote:
>
>> As a new member I wonder if a news list wouldn't
>> be a better avenue. I hope the following has not
>> already been discussed and answered.
>>
>> The goal:
>> ===========
>> Finding a lead-free solder paste for use in
>> SMD rework using PACE hot air workstations,
>> in an educational setting.
>>
>> The problem:
>> ===========
>> I've experiemented very briefly with a sample
>> of solder from EFD which consists of Sn96Ag4
>> "no clean, low residue" paste for its suitability
>> in a Community College soldering program
>> contained within a Computer Technician program.
>>
>> I am looking to use the lead-free paste to replace RMA
>> eutectic paste which we use when teaching
>> installation and removal of 1206, 0805, SOT23, SOICs and
>> PLCC installation using a PACE
>> MBT250 hot air jet soldering station.
>>
>> When I used this paste I noticed some characteristics
>> which would seem to make it unsuitable for
>> my application.
>>
>> 1. The solder seems to melt fine with the hot air
>> jet. The jet is normally set to 900 deg F for eutectic
>> paste use and this works fine for melting the lead-free
>> paste sample I have. (900 deg F translates MUCH
>> lower when it reaches the components.) So the
>> higher melting point does not seem to be a problem
>>
>> 2. When the joint cools, it appears dull and grainy.
>> If this were a eutectic joint, it would fail a visual
>> inspection as it would look like the joint were
>> reheated, overheated, or vibrated during solidification.
>>
>> 3. Wetting action seems minimal. I do not get anywhere
>> near the nice wetting action of eutectic solder. Although
>> it appears barely acceptable for 1206 and 0805 packages,
>> it is horrible for SOIC (gullwing) types. It does not seem to
>> bond well with the lead, and the leads seem to set more
>> on top of the solder rather than "sink" down to the pad
>> with the paste wetting all around the lead. At many points
>> the lead/paste junction looks similar to that on a "cold"
>> joint, although in prying the lead / pad apart, it does seem
>> that an intermetallic bond has formed.
>>
>> I have tried various slow heating stages thinking that the
>> leads were not hot enough, but there was no improvement
>> in the end result.
>>
>> Comments & recommendations?
>> ============================
>> Has anyone successfully used any lead-free paste that
>> gives results similar to eutectic? If so, could you list
>> the actual product and manufacturer so that I could get
>> a sample and test for its suitability in our program.
>>
>> Any comments appreciated.
>>
>> Thanks
>>
>> Dave
>>
>> ==========================
>> Dave Crabbe, Faculty
>> Computer Technician Program
>> N.S.C.C - Burridge Campus
>> 372 Pleasant St, Yarmouth, N.S., B5A 2L2
>> http://cst.burridge.nscc.ns.ca/cst
>>
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